Many vehicles are equipped with safety devices such as airbags, seat belt pretensioners and so forth to protect persons occupying various seats in the vehicle. If a seat is unoccupied or is occupied by a person of a particular size, it may not be necessary to activate a safety device associated with that seat. Furthermore, if a seat is occupied by a person of a particular size the manner in which a safety device is employed may be varied accordingly. One indicator of the size of a seat occupant is his or her weight. In the case of an infant, the combined weight of the infant and an infant safety seat is useful as an indicator of occupant size.